1. Field of the Invention
The present invention relates to a toggle clamping mechanism which is used in an injection molding machine and which prevents the inclination of a movable platen by maintaining the position of a crosshead on a center axis between upper and lower links.
2. Description of the Related Art
As shown in FIG. 6, a mold clamping apparatus of an injection molding machine includes a fixed platen 2 fixed to a base 1, and a movable platen 3 movably disposed parallel to the fixed platen 2. A fixed-side mold half (not shown) is attached to the fixed platen 2, and a movable-side mold half (not shown) is attached to the movable platen 3. The injection molding machine opens, closes, and clamps the mold by advancing and retracting the movable platen 3 with respect to the fixed platen 2.
After the mold is clamped, melted resin is injected from an injection unit (not shown) into the mold. At this time, a sufficiently strong clamping force needs to be applied so that the mold may not be opened by the pressure of injection. Accordingly, a toggle link structure which amplifies a force using toggle links 6 is generally used. In this toggle link structure, a crosshead 7 connected to the movable platen 3 with the toggle links 6 is advanced or retracted by a ball screw 11 rotationally driven by the driving device 10, and the thrust of the crosshead 7 is amplified by the toggle links 6 to generate a clamping force.
With regard to clamping force and mold surface parallelism, to prevent imbalance between upper and lower portions of the mold, generally, the crosshead 7 is disposed on a line connecting the center of the fixed platen 2 and the center of the movable platen 3, and the toggle links 6 are disposed symmetrically about the crosshead 7.
In prior art, as a mechanism for a guide for the crosshead 7, bushings (plain bearings) are inserted into the crosshead 7 and the crosshead 7 is slid along guide rods. In that case, for smooth sliding, a clearance needs to be provided between each bushing and the corresponding guide rod. However, as shown in FIG. 6, due to this clearance, the position of the crosshead 7 is lowered below the center of the guide rod by gravitation, or the crosshead 7 may be inclined by load imbalance between upper and lower portions. In that case, there occurs an angle difference between the upper toggle link 6 and the lower toggle link 6. As a result, the movable platen 3 inclines, and there occurs the problem that a mold mount surface of the movable platen 3 and a mold mount surface of the fixed platen 2 are not parallel. If the mold mount surfaces are not parallel, misalignment occurs between the fixed-side mold half attached to the fixed platen 2 and the movable-side mold half attached to the movable platen 3. Accordingly, problems such as defective shapes of molded products and wear in alignment pins of the mold occur.
Japanese Patent Application Laid-Open No. 2001-260196 discloses a technique for preventing the inclination of a movable platen by employing ball spline shafts in portions of tie rods along which the movable platen slides. In this technique, using ball spline shafts as tie rods, ball spline nuts attached to the movable platen are engaged with (ball-splined to) the ball spline shafts (tie rods). However, this configuration requires as many as four ball spline shafts, thus increasing costs.
Moreover, Japanese Patent Application Laid-Open No. 2009-132087 discloses a technique for reducing the inclination of a movable platen by raising the front sides of guide rods for a crosshead to correct the displacement of the crosshead. However, even when the displacement of the crosshead is compensated using this technique, there is a clearance between each of bushings inserted into the crosshead and the corresponding guide rod, and therefore the crosshead inclines to generate an angle difference between the upper and lower toggle links. As a result, the problem of the inclination of the movable platen occurs.